<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{width: 100px;height: 100px;background-color: paleturquoise;margin-bottom: 10px;}
    </style>
</head>
<body>
    <button onclick="add()">点击将Div变红</button>
    <div></div>
    <div></div>
    <div></div>
</body>
<script>
    let div=document.querySelectorAll("div")
    function add(){
        for(var i=0;i<=3;i++){
            div[i].style.backgroundColor="red"
        }
    }
</script>
</html>